Why Filtration Is the Step Your ICP Results Depend On

Poor filtration quietly damages ICP results long before anyone notices the cause. 

In this article, you’ll learn: 

• Why filtration is a critical but often overlooked step in ICP-OES and ICP-MS sample preparation 

• How unfiltered or poorly filtered samples damage instruments and compromise analytical results 

• How DigiFILTERs, DigiTUBEs™ and the DigiFILTER Manifold work together as an integrated sample preparation workflow 

• How to select the right DigiFILTER membrane and pore size for your application 

• How the DigiFILTER manifold increases efficiency and reduces risk 

Why are ICP instruments sensitive to unfiltered samples? 

In ICP-OES and ICP-MS workflows, filtration is the sample preparation step that removes suspended particulates from a digested sample before that sample reaches the instrument. This protects both the instrument’s components, and analytical accuracy.

The sample introduction system on ICP is designed to transport a liquid sample and generate a stable, fine aerosol. Digested environmental samples such as wastewater, soil extracts, and sediment leachates routinely carry residual particulates the naked eye can’t detect, and suspended solids increase the risk of clogging, unstable aerosol formation, deposition and drift. Consequences of inadequate filtration range from signal instability and elevated backgrounds to physical blockage. Because potential damage builds up gradually, filtration is often treated as a minor step, even though it directly determines whether a result can be trusted.

In many regulated methods for dissolved metals, filtration is a required or method-defining step. This is the case in EPA 6020B, for example, where dissolved constituents must be filtered and acid-preserved before analysis, or EPA 3005A, where dissolved metals are filtered through a 0.45 µm filter at collection before acidification. For other workflows, especially total recoverable or acid-leachable determinations, filtration may occur later, or may not define the reported fraction. But because filtration can alter contamination background and analyte recovery, it should always be method-aligned and validated. In all cases, it’s the point at which months of careful method development either hold or fail.

Where generic filters fall short

Syringe filters are widely used in ICP sample preparation for convenience, but present real limitations for trace-metal analysis:

• Suitability depends heavily on material grade and certification — low-quality filters can introduce contamination and variability at ultra-trace levels.
• They’re designed for single-sample chromatography workflows, not the high-throughput demands of environmental trace metal labs processing dozens of samples per batch.
• Filtration using syringe filters requires manual force through a syringe body, adding a contact point for acid-digested samples.
• The pressure applied in manual syringe filtration varies between operators — a problem that compounds across large batches.

“Syringe filtration works well for individual samples, but many trace metal laboratories process dozens of samples per batch. The time and manual effort required to filter samples one at a time is often the biggest practical challenge.”

Mitra Matloobi, Global Product Manager, ICP Consumables, AnalytiChem Group

Our DigiFILTER range, coupled with DigiTUBE digestion tubes and the DigiFILTER manifold, addresses the shortcomings of syringe filtration.  

DigiFILTERs are designed to form a closed-loop system: the sample is digested, cooled, and filtered in the same tube, minimizing the number of vessels in contact with it, and preserving its integrity throughout.

This matters beyond contamination control. Every transfer step is a potential loss event: adsorption of analytes into vessel walls, evaporative concentration, particulate re-suspension, or spillage of acid-laden solutions that present a safety risk. While the latter would have an obvious, visible influence on workflow and result, the impact of less visible factors should not be underestimated.

The DigiFILTER dual-headed cap is manufactured from virgin polypropylene selected for trace-metal workflows. This matters, because at sub-parts-per-billion (ppb) concentrations, the housing surrounding the membrane isn’t a neutral component, and can contribute to the analyte signal and corrupt blanks. Low-extractable materials, independently verified, are a baseline requirement for reliable trace metal analysis.

DigiFILTERs are available in three membrane types — PTFE (also known by the trade name Teflon®), Metricel, and glass fiber — and pore sizes of 0.2 µm, 0.45 µm, 0.7 µm, 1.0 µm, 10 µm. DigiFILTERs are designed to work directly with 50 ml and 100 ml DigiTUBE digestion tubes — available in 15 ml, 50 ml, and 100 ml volumes — eliminating the sample transfer step between digestion and filtration.

Table: DigiTUBEs vs. generic digestion tubes

How do you choose the right DigiFILTER membrane and pore size?

Selecting the right combination of DigiFILTER membrane and pore size depends on two variables: sample matrix and analytical chemistry.

PTFE is the safest default membrane where the method allows it. PTFE combines excellent acid compatibility with very low extractables, minimizing the risk of the membrane itself contributing to background contamination. Metricel and glass fiber remain useful alternatives, for higher-flow, lower-acid-strength applications, or as a prefiltration step ahead of a finer PTFE membrane where particulate load is high.

Pore size should be selected according to the reported fraction, particle load, and applicable method. A 0.45 µm membrane is widely used to operationally define dissolved constituents in many water methods, but isn’t a universal choice for every environmental water workflow. Samples with high solids content may require a validated prefiltration strategy or another method-authorized approach to prevent premature clogging without changing the intended measurand.

It’s also worth noting that interaction between membrane and analyte isn’t one-directional. As well as potentially introducing metals from the membrane and producing false positives, a poorly chosen membrane can cause analyte adsorption, effectively removing metals from the filtrate and producing false negatives.

The correct choice of filter type and pore size for the DigiFILTER system is led by the method and the matrix; where there’s uncertainty, the AnalytiChem team can advise on selection.

Table: Choosing the right DigiFILTER membrane

How does the DigiFILTER manifold improve efficiency and safety?

The manifold enables vacuum-assisted filtration, replacing the variable manual pressure of syringe-based methods with a controlled, consistent process. Applying uniform vacuum across all positions removes operator-to-operator variability as a source of error.

There’s also a safety benefit. Acid digestion workflows involve the use of concentrated mineral acids, and every manual transfer or handling step adds exposure. The DigiFILTER manifold is designed to operate entirely within a fume hood: once tubes are loaded and DigiFILTERs fitted, filtration proceeds under vacuum with minimal additional manual handling. Acid exposure is reduced, and the process is easier to contain.

Available in four- and twelve-position configurations, the DigiFILTER manifold allows simultaneous filtration of multiple samples without cross-contamination: each DigiFILTER and its corresponding DigiTUBE occupies a separate position with a separate fluid path.

Manifolds can also be configured in series to accommodate larger batches, further increasing cost-efficiency and throughput. Samples from different sources, different clients, or different matrices can be processed simultaneously without risk of carryover.

An integrated filtration system, delivering reliable results

When used together, AnalytiChem’s DigiTUBEs, DigiFILTERs, and manifold form an integrated system, with a single, documented chain from digestion to filtration to dilution. Consistent materials chemistry maintained at every stage turns filtration from a point of risk into a controlled, traceable step your ICP-OES or ICP-MS results can depend on — and one your lab can defend under audit or client scrutiny.

Every advantage in this workflow exists because the system was built for trace-metal throughput, not adapted from single-sample convenience. Where syringe filtration adds a manual contact point and performance variability, the DigiFILTER and DigiTUBE system closes the loop: one tube, one filter, vacuum-driven consistency across every position on the DigiFILTER manifold. A short animation covering our DigiFILTERs, DigiTUBEs, and showing the manifold in action is available to view here.

For a deeper dive into DigiFILTER membrane selection, read the other blogs in this series, How to Choose the Right Filter Membrane for Trace Metal Analysis, and for the full picture of how digestion, filtration, and dilution interact, read Your ICP Results Are Only as Good as Your Sample Preparation.

To discuss your specific filtration requirements, contact our team.